1. Field of the Invention
This invention relates to a novel photostable cyanine dye, to a method for the production thereof, and to an optical recording medium using the dye, particularly a heat mode optical recording medium.
2. Description of Prior Art
Cyanine dyes are used in various fields. Typical application in which they are used include silver halide photographs, dye lasers, electrophotographic sensitizers, and optical recording media.
Especially, cyanine dyes of the indolenine type are ideally suitable for optical recording media for reasons such as high solubility and low crystallinity (Japanese Patent Application SHO 57(1982)-134397, SHO 57(1982)-134170, SHO 57(1982)-17776, etc.). Unfortunately, cyanine dyes of the indolenine type have a disadvantage that they are easily discolored because of their poor resistance to light.
During the course of my study devoted to the solution of the problem of the discoloration of cyanine dye used in the recording layer of a heat mode optical recording medium in the form of a film cured by repeated exposure of the recording layer to the reading light (degradation by reading), I have already proposed highly satisfactory solutions of this problem of degradation by reading by the incorporation of quenchers of the bisphenyl dithiol type (Japanese Patent Application SHO 58(1983)-163,080, SHO 57(1982)-166,832, SHO 57(1982)-168,048 and SHO 57(1982)-7,776).
For the improvement of the resistance of cyanine dye to light, however, the method depending on the mixture of the cyanine dye with a quencher is effective only to a limited extent because the molecules of the dye are not necessarily joined as strongly to the added quencher as expected.
The cyanine dye is generally in the form of a cation coupled with an acid anion and the quencher is generally in the form of an anion coupled with an onium cation. The mixture, therefore, entails the possibility that, owing to the acid anion and the onium cation, the dye and the quencher will undergo hydrolysis and bring about such adverse effects as degradation of the resistance to moisture, for example.
Further, the acid anion and the onium cation have an effect of diluting the dye and causing the reflectivity of the film to fall below the value proper to the cyanine dye with the result that the reproducing S/N ratio decreases.
The optical recording medium is such that the medium itself is not brought into contact with the writing or reading head and, therefore, is characterized by enjoying freedom from deterioration caused by wear. This strong point has encouraged various studies directed to development of optical recording media.
In the optical recording media, those of the heat mode are commanding particular interest of researchers because they do not need darkroom treatment for development.
The heat mode optical recording medium is an optical recording medium which utilizes the recording light in the form of heat. For example, the pit-forming type optical recording medium is based on the procedure of writing given information on the medium by forming small holes called pits by the removal of part of the medium with a recording light such as a laser and reading the recorded information by sensing these pits with a reading light.
In the pit-forming type medium of this operating principle, particularly of the kind using a semiconductor laser permitting miniaturization of device, the recording layer is formed principally of a material composed mainly of Te.
In recent years, numerous proposals and reports have come to cover those media which use recording layers of organic materials composed mainly of dyes in the place of Te because Te type materials are harmful and require more thorough sensitization and the cost of production must be lowered.
Among the organic materials in the recording layers which require the He-Ne laser for the formation of pits, those using squalirium dyes [Japanese Patent Application Laid-open SHO 56(1981)-46,221 and V. B. Jipson and C. R. Jones.: J. Vac. Sci. Technol., 18, (1) 105 (1981)] and those using metal phthalocyanine dyes (Japanese Patent Application Laid-open SHO 57(1982)-82,094 and SHO 57(1982)-82,095) are included.
Another such organic material which similarly requires the He-Ne laser uses a metal phthalocyanine dye (Japanese Patent Application Laid-open SHO 56(1981)-86,795).
These optical recording media invariably have their recording layers containing respective dyes deposited in the form of film by vacuum deposition and, in this sense, do not differ much from those using Te in terms of the production of recording medium.
Generally, since the laser beam impinging on the vacuum deposited film of dye has low reflectance, the recording medium using this film fails to obtain an ample S/N ratio in the usual widely employed method which derives read signals from variations (decreaes) in the amount of reflected light caused by bits.
When the optical recording medium is produced in the so-called air-sandwich type construction having a recording layer opposed to and joined fast to a transparent substrate so that writing and reading of information is effected through the substrate, the recording layer can be protected from deterioration without lowering the writing sensitivity and the recording density can be improved. Even this recording and reproducing method is impraticable with the vacuum deposited film of dye.
This is because, in the ordinary substrate made of transparent resin, the refractance has a fair value (1.5 in the case of polymethyl methacrylate) and the surface reflectance has a fairly large value(4% in the same resin) and the reflectance on the recording layer through the substrate is not more than about 6.0% in the case of polymethyl methacrylate, for example, and the recording layer which exhibits only a low reflectance, therefore, does not permit reliable detection of variations in the amount of reflected light.
For the purpose of improving the S/N ratio of reading of the recording layer formed of a vacuum deposited film of dye, generally a vacuum deposited reflecting film such as of A1 is interposed between the substrate and the recording layer.
In this case, the vacuum deposited reflecting film is intended to increase the reflectance and improve the S/N ratio. By the formation of pits, the reflecting film is exposed to increase the reflectance or, at times, the reflecting film is removed to decrease the reflectance. As a matter of course, the recording and reproduction of information cannot be effected by the light passed through the substrate.
The recording media in which a recording layer composed of a dye and resin is formed by the spreading technique have also been disclosed such as, for example, the medium using a recording layer formed of IR-132 dye (made by Kodak) and polyvinyl acetate (Japanese Patent Application Laid-open SHO 55(1980)-161,690), the medium using a recording layer formed of 1,1'-diethyl-2,2'-tricarbocyanine-iodide and nitrocellulose (Japanese Patent Application Laid-open SHO 57(1982)-74,845), and the medium using a recording layer formed of 3,3'-diethyl-12-acetylthiatetra-carbocyanine and polyvinyl acetate.
These optical recording media also require interposition of a reflecting film between the substrate and the recording layer and, therefore, are as defective as those involving use of a vacuum deposited film of dye in respect that recording and reproduction are not obtained by the light projected from behind the substrate.
To realize an optical recording medium possessing a recording layer of an organic material which permits recording and reproduction by the light passed through the substrate and exhibits interchangeability with a medium possessing a recording layer of a Te-based material, the organic material itself is required to exhibit high reflectance.
In a very small fraction of the optical recording media so far developed, the recording layers of organic materials incorporated therein without interposition of any reflecting layer exhibit high reflectance.
There have appeared publications purporting that a vacuum deposited film of vanadyl phthalocyanine exhibits high reflectance [P. Kivits et al., Appl. Phys. Part A 26 (2) 101 (1981), Japanese Patent Application Laid-open SHO 55(1980)-97,033]. Despite the high reflectance which is possibly ascribable to the high subliming temperature of the compound, the film is noted to possess poor writing sensitivity.
It has been reported that cyanine dyes and merocyanine dyes of the thiazole type or quinoline type exhibit high writing sensitivity [Yamamoto et al.: Glossary of Manuscripts for the 27the Meeting of Applied Physics, lp-p-9 (1980)]. An invention based on this publication has been proposed under Japanese Patent Application Laid-open SHO 58(1983)-112,790 These dyes are impracticable, however, because the dyes, particularly when they are converted into applied layers of the form of film, exhibit low degrees of solubility in solvents, undergo crystallization readily, offer very poor stability to resist the reading light, and quickly discolor.
In view of the true state of affairs described above, the inventors have already proposed a single-layer film of a cyanine dye of the indolenine type which has high solubility in solvents, undergoes crystallization minimally, offers thermal stability, and enjoys high reflectance (Japanese Patent Application SHO 57(1982)-134,397 and SHO 57(1982)-134,170).
They have also proposed improvement of solubility and prevention of crystallization in the cyanine dyes of the indolenine type or the thiazole type, quinoline type, selenazole type, etc. by the incorporation of a long-chain alkyl group in the molecules thereof (Japanese Patent Application SHO 57(1982)-182,589 and SHO 57(1982)-177,776).
They have further proposed addition of transition metal compounds as quenchers to cyanine dye for the enhancement of photostability and particularly the prevention of decolorization by light (degradation by reading) (Japanese Patent Application SHO 57(1982)-166,832 and SHO 57(1982)-168,048).
Generally cyanine dyes are coupled with anions such as, for example, ClO.sub.4.sup.-. Ordinary transient metal chelate compounds as quenchers are coupled with cations such as, example, ammonium ion.
When cyanine dyes and quenchers described above are combined in recording layers, therefore, such unnecessary paired anions and paired cations are inevitably contained in the recording layers, and, on hydrolysis, they tend to produce acids and alkalis. Thus, these recording layers have posed a problem regarding the resistance to moisture. Further, the inclusion of such unnecessary moieties results in an increase of the molecular weights and a proportional decrease in the absorbance and reflectance per unit weight. Thus, the recording layers prove disadvantageous in term of enhancement of sensibility.